Capacitively tuned reflector antenna



' Feb. 4, 1969 R. c. FENWICK 3,426,352

CAPACITIVELY TUNED REFLECTOR ANTENNA Filed Aug. 26, 1966 Sheet of I f l/7 FIG I; {/0

ll/ l3 4-l2 GENERATOR 28 27 26 29 3/ 3 m r 1 v I Al I I Al (2 I 21 TI C2/- 22 /0 FIG 2 [/7] I l I I GENERATOR 32$: FIG 3 -s GENERATOR INVENTOR.4 RlCHARD C. FENWICK L m 6 ATTORNEY Feb. 4, 1969 R. c. FENWICK 3,426,352

CAPACITIVELY TUNED REFLECTOR ANTENNA Filed Aug. 26, 1966 Sheet 2 of 2 rLI 1 V i m c c C 41 49 C3 IHI C2146 f 4 C4I 221D- C2 1 h 3 H4 3 I .L F-47 INCHES 3 s 6% INCHES H s INCHES K=.666

(I g 2 ELE6M0ENT ELE4M6ENT ELEMENT ELEMENT L 57 50 65 80 95 HO I- I I II 200 2|5 230 245 FREQUENCY, MC

FIG 6 IN VENTOR. RICHARD C. FENWICK BY 1 J t (.C f ATTORNEY UnitedStates Patent 9 Claims This invention relates in general to antennasand, in particular, to an antenna which may be tuned over a wide bandwith a low voltage standing wave ratio and unidirectional radiationpattern.

It is an object of this invention to provide an antenna capable of beingtuned over a broad band of frequencies and which gives a good voltage tostanding wave ratio over the frequency range.

Another object of this invention is a provision for an improvedmultielement broadband antenna with electrically small dimensions.

A feature of this invention is found in the provision for a plurality ofcapacitively loaded multiple antenna elements which are so connected toform a composite antenna which is tunable over a broad frequency rangeand which has a good standing wave ratio.

Further objects, features, and advantages of this invention will becomeapparent from the following description and claims when read in View ofthe accompanying drawings, in which:

FIGURE 1 illustrates a capacitively tuned antenna;

FIGURE 2 shows a pair of antennas such as shown in FIGURE 1 connectedtogether;

FIGURE 3 shows a modification of the feed system of the antenna inFIGURE 2;

FIGURE 4 shows the novel multielement antenna of this invention;

FIGURE 5 is a side view of the antenna; and,

FIGURE 6 is a plot of the voltage standing wave ratio versus frequencyfor a particular antenna.

FIGURE 1 illustrates an antenna which is the subject matter of mycopending application entitled Capacitively Tuned Electrically SmallAntenna filed Dec. 27, 1965, Ser. No. 517,111, now abandoned. Suchantenna comprises an element mounted above a ground screen or reflectingscreen 10 through which coaxial feedline 12 extends. Line 12 has anouter conductor 13 and an inner conductor 14. A suitable radio frequencygenerator 11 is connected to the coaxial cable 12. A relatively shortfeed portion 16 extends outwardly from the ground screen 10 and connectsto the inner conductor 14 of the coaxial cable. A matching inductor 17is connected between the ground screen 10 and the portion 16. Aconducting element 18 is bent at right angles to the ground screen andconnects to the portion 16. A variable capacitor 19 is connected at theother end of the conductor 18 and a capacitor plate C is attached to theother side of the variable condenser 19.

As pointed out in the copending patent application referred to above,such an antenna is tunable over a wide frequency band and has a lowvoltage standing wave ratio and radiates primarily as a verticalmonopole parallel to the ground plane. By combining a pair of theseantennas into a balanced structure over a ground plane as shown inFIGURES 2 and 3, an antenna with completely different characteristics isobtained.

In FIGURE 2, the generator 11 is connected through a balanced feedlinehaving conductors 21 and 22 to conductors 23 and 24. A matching inductor26 is connected between the end feedlines 21 and 22. The conductors 23and 24 extend parallel to the ground screen 10. Condenser 27 isconnected to the other end of the conductor 23 and a capacitive plate 28is connected to the other side of the condenser 27. Likewise, a variablecondenser 29 is connected to the other end of conductor 24 and capacitorplate 31 is connected to the other side of the condenser 29.

FIGURE 3 illustrates a modification of the antenna of FIGURE 2 wherein acoaxial cable 32 is connected to the generator 11 and has its outerconductor 33 connected to ground and to the end of element 24 and itsinner conductor 34 is conected to element 23. A stub coaxial cable 36has its outer conductor 37 connected to the element 23 and joins thecoaxial cable 32 at a point intermediate its ends.

The antennas of FIGURES 2 and 3 are the same except for the type of feedand these antenas give a unidirectional radiation pattern orientedperpendicular to the ground screen or reflecting screen. Radiationparallel to the reflecting screen is negligible due to the out-of-phasecurrents I and I in the feedlines 21 and 22. However, the antennas ofFIGURES 2 and 3 do not give good voltage standing wave ratios when tunedover a wide band because of the unfavorable variation of radiationresistance with frequency. It has been discovered that this limitationcan be overcome by combining two or more elements such as shown inFIGURES 2 and 3 into a composite antenna in which each element has aproper length to cover a relatively small frequency band.

FIGURE 4 illustrates such an antenna from the top view. The groundreflecting screen 10 lies in the plane of the drawing and a generator 11has a pair of feedlines 40 and 41 which extend outwardly from the groundplane. A matching inductor 42 is connected across the ends of feedlines40 and 41 and a first pair of antenna element feedlines 43 and 44 extendparallel from the ends of feedlines 40 and 41 as shown. A first highfrequency antenna element 46 is connected to feedlines 43 and 44.Antenna 46 comprises a pair of radiating elements 47 and 48 whichextend, respectively, outwardly from feedlines 43 and 44 and variablecapacitors O and C are connected to the end of leads 47 and 48.Capacitor plates C and C are attached to the other sides of condenser Cand C A lower frequency antenna 49 is mounted further from the end offeedlines 40 and 41 on feedlines 43 and 44 and comprises a pair ofantenna elements 50 and 51 with variable capacitors C and C at the endsof elements 50 and 51 and capacitive plates C and C connected to theother sides of these condensers.

Also connected to the end of feedlines 40 and 41 are crossover phasereversing leads 52 and 53 which connect to feedlines 54 and 56 whichextend in a direction opposite from feedlines 43 and 44 from feed points40 and 41. The lowest frequency antenna element 57 is connected to theend of feedlines 54 and 56 and comprises radiating elements 58 and 59which have variable capacitors C and C at their ends and capacitiveplates C and C connected to the condensers. A relatively high frequencyantenna element 60 is also connected to feedlines 54 and 56 andcomprises radiating elements 61 and 62 with capacitors C and C connectedto their ends, respectively, and with capacitive plates C and Cconnected to these capacitors. It is to be noted that the antenna ofFIGURE 4 illustrates four antenna elements of the type shown in FIGURES2 and 3 which form a composite antenna that may be used to radiate orreceive energy over a broad frequency spectrum with a low voltagestanding wave ratio. The length and spacing and usually the width ofeach of the elements are related to those of the adjacent band elementby a scaling factor K. In other words, if the length of each of theantenna elements 46, 49, 57 and 60 is indicated by L L L and Lrespectively, the following relationships exist:

If the distance from the feedlines 40 and 41 to the radiating elementsof each antenna is designated S S S and S for each of the antennas 46,49, 57 and 60, respectively, the following relationships also hold:

K is equal to f /f where f is the center frequency to which a particularone of the antenna elements can be tuned and f is the center frequencyto which the next higher frequency antenna element can be tuned. Ingeneral,

fow

K fo( where n is the element number. The dimensions of the end plates Cand C C and C C and C C and C do not generally scale in this manner butare made just large enough so that a given element can be resonated atthe lowest frequency in its band with its tuning variable capacitors attheir maximum values.

The antenna elements are spaced about the feed point so that element tofeed point spacing 5 may be as small as possible while the spacingbetween elements of adjacent frequency band antenna elements may be asgreat as possible. Crossing over (transposing) the feedline asillustrated by leads 52 and 53 is preferable, but is not required foroperation.

FIGURE 5 is a view taken at right angles to FIGURE 4 and illustrates amodification which may be made wherein the heights of the end plates C CC C C C C and 0., were not held constant but were related to one anotherby the scaling factor K. In other words,

This is preferred, but not required.

FIGURE 6 is a plot of voltage standing wave ratio versus frequency for aparticular antenna with four elements and it is to be noted thatgenerally the standing wave ratio over the frequency range from 50 to230 megacycles is less than 1.5. The voltage standing Wave ratio isbelow 2 over a 5 :1 frequency band.

It is seen that this invention provides a driven antenna over a groundscreen or reflector in a broadside radiation configuration which can betuned by means of variable capacitors over a wide band, whilemaintaining a good voltage standing wave ratio and a good radiationpattern. The antenna may be spaced close to the reflector and in oneembodiment has a simple means to feed the balanced antenna with anunbalanced transmission line.

Although the antenna has been described with respect to particularembodiments thereof, it is not to be so limited, as changes andmodifications may be made therein which are within the spirit and scopeof the invention as defined by the appended claims.

I claim:

1. A broadband antenna comprising a ground plane, a pair of feedlinesextending outwardly from the ground plane, a second pair of feedlinesconnected to the ends of the first pair of feedlines and extendingsubstantially parallel to the ground plane, a first antenna elementconnected to the second pair of feedlines and comprising first radiatingelements, a first pair of variable capacitors connected to the ends ofthe radiating elements, a first pair of capacitor plates connected tothe first pair of variable capacitors, a second antenna elementconnected to the second pair of feedlines and comprising secondradiating elements, a second pair of variable capacitors connected tothe ends of the second radiating elements, and a second pair ofcapacitor plates connected to the second pair of variable capacitors.

2. In apparatus according to claim 1, a third pair of feedlinesconnected to the ends of the first pair of feedlines and extendingsubstantially parallel to the ground plane and a third antenna elementconnected to the third pair of feedlines and comprising third radiatingelements, a third pair of variable capacitors connected to the thirdradiating elements, and a third pair of capacitor plates connected tothe third pair of variable capacitors.

3. In an antenna according to claim 2, additional radiating elementsconnected to the feedlines and each element having variable condensersattached to their ends and capacitor plates attached to each of thevariable condensers.

4. In an antenna according to claim 3, wherein all of the radiatingelements and their associated condensers are tuned to different centerfrequencies.

5. In apparatus according to claim 4, wherein the lengths of eachradiating element bears a relationship to its adjacent frequencyradiating element determined by a constant scaling factor.

6. In apparatus according to claim 4, wherein all of the radiatingelements are spaced from the first pair of feedlines by distances whichare related to the center frequencies radiating elements that areresonant at adjacent frequencies.

7. In apparatus according to claim 3, wherein the third pair offeedlines are transposed adjacent their junction with the first pair offeedlines.

8. In apparatus according to claim 4, wherein the capacitor plates ofvarious radiating elements are spaced varying distances from the groundplane.

9. A broadband antenna comprising a ground plane, a pair of feedlinesextending outwardly from the ground plane, an antenna element comprisingradiating elements each connected to one of the feedlines, a pair ofvariable capacitors connected to the ends of the radiating elements, anda pair of capacitor plates connected to the pair of variable capacitors.

References Cited UNITED STATES PATENTS 3,151,328 9/1964 Boyer 343-752ELI LIEBERMAN, Primary Examiner.

U.S. Cl. X.R.

1. A BROADBAND ANTENNA COMPRISING A GROUND PLANE, A PAIR OF FEEDLINESEXTENDING OUTWARDLY FROM THE GROUND PLANE, A SECOND PAIR OF FEEDLINESCONNECTED TO THE ENDS OF THE FIRST PAIR OF FEEDLINES AND EXTENDINGSUBSTANTIALLY PARALLEL TO THE GROUND PLANE, A FIRST ANTENNA ELEMENTCONNECTED TO THE SECOND PAIR OF FEEDLINES AND COMPRISING FIRST RADIATINGELEMENTS, A FIRST PAIR OF VARIABLE CAPACITORS CONNECTED TO THE ENDS OFTHE RADIATING ELEMENTS, A FIRST PAIR OF CAPACITOR PLATES CONNECTED TOTHE FIRST PAIR OF VARIABLE CAPACITORS, A SECOND ANTENNA ELEMENTCONNECTED TO THE SECOND PAIR OF FEEDLINES AND COMPRISING SECONDRADIATING ELEMENTS, A SECOND PAIR OF VARIABLE CAPACITORS CONNECTED TOTHE ENDS OF THE SECOND RADIATING ELEMENTS, AND A SECOND PAIR OFCAPACITOR PLATES CONNECTED TO THE SECOND PAIR OF VARIABLE CAPACITORS.